Trunked radio communication systems are known. The Smartnet and Privacy Plus systems offered by Motorola, Inc., and the 16 PLUS and Sweet Sixteen systems offered by General Electric, provide an infrastructure that includes a plurality of repeaters that each operate on different frequencies, wherein at least one of these frequencies generally functions as a dedicated control channel for supporting the transmission and reception of system control information, such as requests from communication units for communication resources to support desired communications, and communication resource allocation information. Other systems, such as the Clear Channel system offered by E. F. Johnson, provides such system control information in a subaudible portion of the channel as transmitted by a plurality of their repeaters. Regardless of the system, this system control information typically assumes the form of tone or digitally encoded messages that are intended for the appropriate logic section of the system infrastructure and/or the radio, and not the ears of the user.
Encrypted trunked communication systems are also known. In such systems, the verbal messages of the users are encrypted by one form or another to render unauthorized eavesdropping more difficult. In a similar manner, data messages have been encrypted on trunked systems, wherein the data comprises user information, and not system control information as described above.
As the sophistication of electronic eavesdroppers has grown, a commensurate need to render more difficult their unauthorized activities has grown. It now becomes more important to provide for the encryption of system control information, sometimes in conjunction with encrypted user messages, and sometimes not. Preferably, the encryption technique employed would be generally compatible with much of the trunking infrastructure that is currently installed and operational. Furthermore, the encryption methodology should preferably accommodate a variety of encryption approaches, and should support flexibility and ease of revision.